The auxiliary oil system of an aircraft gas turbine engine is a closed circuit fluid system that circulates oil through the integrated-drive generator system of the engine as well as oil-air and oil-fuel heat exchangers. Like other closed circuit systems, the auxiliary oil system normally experiences a lack of oil consumption. Consequently, service checks are generally made infrequently and then primarily to determine whether there has been an oil depletion. The failure to frequently service such a closed circuit fluid system can lead to the creation of sludge, a deterioration in the fluid, e.g., an increase in oil acidity, or fluid depletion due to leakage or vapor loss, among other problems. Furthermore, these problems can be exacerbated as the operating temperature regime of the closed circuit system increases.
Engine subsystems serviced by the auxiliary oil system, such as the integrated-drive generator, may be adversely affected by the above-recited problems. As a result, such subsystems may fail over a period of many hours of service use. Accordingly, failure potential is dependent at least in part upon the peak and average temperatures reached by the oil. It is possible to reduce the possibility of break down by decreasing the operating temperature of the auxiliary system through the use of larger oil-fuel heat exchangers and by increasing oil-air heat exchanging. Obviously, the former is undesirable on an aircraft where space can be at a premium and weight is an important consideration, and the latter results in decreased engine efficiency. Conversely, the system could be operated at higher temperatures and engine efficiency could be increased if the frequency of service and, thus, complete oil replacement in the auxiliary system were increased. More frequent service would increase labor costs, however.
In contrast to the auxiliary oil system, the main oil system of a gas turbine engine is continuously monitored and serviced because some amount of engine oil is normally consumed during engine operation. Thus, unlike the closed circuit auxiliary oil system, the main engine oil system does not suffer from the previously mentioned deficiencies. While it may seem desirable to wed these systems, they presently cannot be joined during operation because particle contamination present in one system could damage the components serviced by the other system and, ultimately, could lead to an engine shutdown.